Thin film deposition using a plasma and etching using a plasma have been employed in many technical fields. A capacitive-coupling-type plane-parallel-plate plasma-enhanced chemical vapor deposition apparatus (hereinafter referred to simply as a “PECVD apparatus”) and a plasma etching apparatus have been used for plasma thin film deposition and for plasma etching. A thin film is deposited by making SiH4 (silane) and such a gas for film deposition (or a gas for film formation: hereinafter referred to as a “film deposition gas”) flows into a vacuum chamber in any of the apparatuses described above. Etching is performed by making CF4 (fluoride gas), and causing such an etching gas to flow into a vacuum chamber in any of the apparatuses described above.
Now, with reference to FIG. 12, the manufacture of a thin film solar cell by the plasma-enhanced CVD method will be described by way of example in connection with the formation of a thin film containing silicon as the main component thereof (hereinafter referred to simply as a “Si thin film”) using a film deposition gas containing SiH4 as the main component thereof. FIG. 12 is a cross-sectional view schematically showing a conventional PECVD apparatus.
Referring to FIG. 12, the conventional PECVD apparatus includes a vacuum chamber 201; a high-frequency-wave electrode 210 positioned in vacuum chamber 201 and connected to high-frequency electric power fed from a high-frequency electric power supply (RF) 205, and an earth electrode 211 positioned in vacuum chamber 201 and connected to the ground potential. It is not always necessary to connect earth electrode 211 to the ground potential. Earth electrode 211 may include a mechanism for feeding DC electric power or high-frequency electric power thereto depending on the purposes thereof. Earth electrode 211 includes a mechanism for positioning a substrate 212 thereon and a heating mechanism (hereinafter referred to as a “heater”) 204 therein for heating substrate 212. Without difficulty, substrate 212 may be positioned at an arbitrary position in vacuum chamber 201 such as on high-frequency-wave electrode 210. Heater 204 may be positioned at an arbitrary position in vacuum chamber 201 or omitted from the CVD apparatus.
A thin film is deposited by the following steps. First, the inside of vacuum chamber 201 is evacuated to certain degrees of vacuum through an exhaust system (an exhaust line 203 such as a vacuum pump and an exhaust pump). Then, substrate 212 is heated with heater 204, if necessary. Immediately after vacuum chamber 201 is evacuated, moisture and such other impurities are absorbed very often by the inner surface of vacuum chamber 201 and the surface of substrate 212. If a thin film is deposited in the state in which these impurities are not sufficiently degassed, a large amount of impurities are contained in the thin film, impairing the film quality. To facilitate degassing the inside of vacuum chamber 201, a gas is sent into vacuum chamber 201 through a gas feeder line 202 and the inside of vacuum chamber 201 is heated (baked) in the state in which the inside of vacuum chamber 201 is kept at the predetermined degrees of vacuum by a not shown pressure controller and gas exhaust line 203. A gas such as H2 that exhibits a relatively high thermal conductivity, an inert gas such as He and Ar, and a film deposition gas made to flow for film deposition are employed for the gas that is made to flow during the baking performed in the vacuum chamber. The temperature of substrate 212 during the baking is set sometimes to be higher than the temperature of substrate 212 during the actual film deposition to facilitate degassing and to reduce the amount of degassing during film deposition.
After the degassing is completed, substrate 212 is set at the temperature at which film deposition is performed. The inside of vacuum chamber 201 is kept under an appropriate pressure by flowing a gas mixture containing several kinds of film deposition gases mixed at an appropriate mixing ratio depending on the films to be deposited. Then, a thin film is deposited on substrate 212 with a plasma 206 generated between high-frequency-wave electrode 210 and earth electrode 211 by feeding electric power to high-frequency-wave electrode 210. Plasma 206 is generally called a “low-temperature plasma”.
The selections of the conditions for the film deposition described above are very important for determining the thin film quality, the film deposition speed, and the film thickness uniformity in the effective film deposition area. The film deposition conditions include hardware configuration conditions such as the spacing between high-frequency-wave electrode 210 and earth electrode 211 (hereinafter referred to as the “electrode spacing”) and the frequency of high-frequency electric power supply 205. The conditions of film deposition for depositing a silicon thin film include the hydrogen dilution rate that is a flow rate ratio of a film deposition gas SiH4 and a diluent gas H2, the temperature of substrate 212, the pressure at which film deposition is performed, the frequency of high-frequency electric power supply 205, the level of supplied electric power, and the spacing between high-frequency-wave electrode 210 and earth electrode 211. These conditions affect each other in determining the thin film properties.
Japanese patent publication JP P 2004-253417 A describes one way of selecting the film deposition conditions. According to that publication, a photoelectric converter device exhibiting excellent photoelectric conversion characteristics is obtained by depositing a photoelectric converter layer at the hydrogen amount ratio (SiH2/SiH), which is a ratio of the amount of hydrogen in the SiH2 bondings to the amount of hydrogen in the SiH bondings in the photoelectric converter layer, set at 0.3 or smaller and at the peak-to-peak voltage Vpp, which is the average value of the high-frequency voltages applied between the electrodes, set at 300 V or lower (cf. FIGS. 3 and 5 and the descriptions with reference thereto in the Japanese patent publication JP P 2004-253417 A). Japanese patent publication JP P 2004-253417 A describes also the existence of a strong correlation between the peak-to-peak voltage Vpp and the photoelectric conversion characteristics, and that a photoelectric converter device exhibiting better photoelectric conversion characteristics is obtained at a lower peak-to-peak voltage Vpp (preferably Vpp≦200 V).
Japanese patent publications JP P 2000-164520 A, JP, P 2001-257098 A, and JP P 2002-313743 A describe means for improving the thickness uniformity in the effective film plane. The means described in Japanese patent publication JP P 2000-164520 A includes high-voltage variable capacitors disposed on the high-frequency-electric-power-supply side of a high-frequency-wave electrode for generating a plasma and on the opposite side thereof. The improvement in film thickness uniformity described in Japanese patent publication JP P 2000-164520 A makes the plasma potential uniform in film deposition by changing the high-frequency-wave phase using the high-voltage variable capacitors. In Japanese patent publication JP P 2001-257098 A, the voltage distribution in the discharging electrode is changed by changing the phase and/or the difference in phase between multiple feeding points of the high-frequency electric power with time. In Japanese patent publication JP P 2002-313743 A, the variations in the film thickness are reduced by making the phases of a high-frequency voltage different on the adjacent small electrodes. Each of these four Japanese patent publications employs a technique for making the plasma distribution uniform, which exhibits a correlation with the film thickness uniformity by changing the apparatus configurations for film deposition.
Although the peak-to-peak voltage Vpp has been one of the film deposition conditions to be selected, few have proposed and described the use of the peak-to-peak voltage Vpp for improving the film thickness uniformity. Japanese patent publication JP P 2004-253417 A describes embodiments that apply the power supply frequencies of 13.56 MHz and 27.12 MHz for manufacturing a solar cell on a film substrate having a film deposition area of 40 cm×80 cm. However, the wavelength of the electric power supply frequency and the size of the electrode become almost the same when the electrode area is widened to be on the level of 1 m×1 m or when the power supply frequency is raised. As the wavelength of the electric power supply frequency and the electrode size approach each other, the uniformity of the film thickness in the effective film plane may be impaired.
The conventional means for increasing the film thickness uniformity do so by improving the apparatus configurations for film deposition related, for example, to the electric power supply scheme. Therefore, no attention has been paid to the measurable quantities related to the change in uniformity of film thickness caused by a change in the conditions of film deposition.
Means are known for measuring the plasma distribution exhibiting a correlation with the uniformity of film thickness. For example, a probe method that inserts a probe into a plasma for measuring the plasma distribution, or a spectroscopic method may be used. However, the probe method causes large errors in the atmosphere of the film deposition. The spectroscopic measurement requires a large-scale measuring system for measuring the uniformity of film thickness over a wide area. Therefore, the conventional means for measuring the plasma distribution exhibit respective limitations in measuring the plasma distribution uniformity over a wide area. It has been difficult to apply any of the conventional means for measuring the plasma distribution to a manufacturing apparatus in practical use on a production line except for a manufacturing apparatus that is actually designed with a means for measuring the plasma distribution taken into consideration. When the apparatus is designed in consideration of the plasma measurement, the manufacturing costs of the apparatus soar.
When the peak-to-peak voltage Vpp on the high-frequency-wave electrodes is considered, the absolute value of the peak-to-peak voltage Vpp and the distribution of the peak-to-peak voltage Vpp are different, not only from lot to lot but also between the early stage and the late stage of lot manufacture. Since manual cleaning is performed for every lot, the variations are caused in the peak-to-peak voltage Vpp and the absolute value thereof at the time when the manual cleaning is performed. It is considered that the variations of the peak-to-peak voltage Vpp and the absolute value thereof, as between the early stage and the late stage of lot manufacture, are due to the increasing amount of powder on the wall of the film deposition room with repeated film deposition operations.
In view of the foregoing, it would be desirable to obviate the problems described above. In detail, it is a first object of the invention to provide a plasma control method that facilitates obtaining relatively simply and easily a means (guideline) for increasing the film thickness uniformity in consideration of the measurable quantities related to change in the film thickness uniformity caused by the changes in the conditions of film deposition.
It is a second object of the invention to provide a plasma control method that facilitates reduction of measurement errors, not with a large-scale measuring system, but with a low-cost control apparatus.
A third object of the invention is to provide a plasma control method that facilitates obtaining very uniform film characteristics and improving the throughput of manufacture, in response to several variations in characteristics. The variations would include, for example, the absolute value and distribution of the peak-to-peak voltage Vpp from lot to lot, and characteristics that depend on the time of lot manufacture, for example, whether it is an early stage of lot manufacture or a late stage of lot manufacture.
A fourth object of the invention is to provide a plasma control method that does not impair the uniformity of thickness in the film plane. The thickness in the film plane should be uniform even when the electrode area is widened to be on the level of 1 m×1 m or the power supply frequency is raised.